The field of the invention is that of air guide boxes. More specifically, the field is that of air guide boxes for stabilizing a paper web.
Air guide boxes are usable for various applications. In conjunction with paper making machines, an air guide box may be used to guide:
(a) a paper web, which together with a backing belt, running from a roll (e.g., a drying cylinder) to an adjacent roll (e.g., a successive drying cylinder);
(b) a paper web running without a backing belt from a roll or a nonrotating bar to a second roll or a second nonrotating bar;
(c) a paper web being wound on a paper roll.
Also, those air guide boxes are usable outside the paper machine technology whenever webs, such as textile webs, are subjected to a similar handling.
An example of an air guide box is described in U.S. Pat. No. 4,628,618 (Virta). Provided on this air guide box, endwise, are air blocking strips formed by side plates or ejector devices. The ejector devices consist of a nozzle arranged at a distance from the material web. That web consists of backing belt and adhering paper web. The nozzle blows at entering leakage air to keep the leakage air flow pushing into the gap between the first wall of the air guide box, the guide wall, and the material web at a minimum.
More generally, the purpose of the air blocking strips is to enable the adjustment, i.e., control of the flow conditions in the air gap. Thus, the vacuum profile between the air guide box and the material web is controlled, as far as possible solely by blowing air supply into the air guide box.
The basic disadvantage of the simple mechanical strips is wear, considering that the backing belts opposite the strips have edges where they are abutted together. Furthermore, mechanical strips have the disadvantage that the marginal flutter of the material web allows undefined leakage flow.
Ejector devices have the disadvantage of blowing against the run direction of the material web and at the marginal area of the web, thereby possibly even furthering the endwise flutter. The energy of the drive jet issuing out of the ejector device needs to be determined directly in contingence on the leakage air amount which is to be expected due to the distance between ejector and web.
The prior ejector devices thus require relatively large amounts of air which, additionally, must "push" against the leakage air with a relatively large kinetic energy.